1. Field of the Invention
The present invention relates to hydrogen absorbing Ni,Zr-based alloys and sealed rechargeable alkaline batteries or cells containing such alloys as an active material for their negative electrodes.
2. Description of Prior Art
Generally, sealed Ni-hydrogen rechargeable batteries include a negative electrode provided with a hydrogen absorbing alloy as an active material, an Ni positive electrode, a separator and an alkaline electrolytic solution. The hydrogen absorbing alloys constituting the negative electrode are desired to have the following characteristics, for example.
(a) They have high capability to absorb and release hydrogen. PA1 (b) They show a relatively low equilibrium hydrogen dissociation pressure (which corresponds to the plateau pressure on PCT curve at a temperature in the vicinity of room temperature) as low as not higher than 5 atm. PA1 (c) They exhibit high corrosion resistance and high durability or resistance to deterioration or aging. PA1 (d) They show high hydrogen oxidizing capability (or catalytic activity). PA1 (e) They are hardly converted to fine powder when absorption and release of hydrogen are repeated. PA1 (f) They cause no or less environmental pollution. PA1 (g) Their cost is low.
As is well known in the art, sealed Ni-hydrogen rechargeable batteries using as an active material such hydrogen absorbing alloy as having the above-listed characteristics could exhibit various desirable characteristics such as large discharge capability, long lifetime of repeated cycle of charging and discharging, excellent rapid charging and discharging characteristics, and low self-discharge.
Therefore, research and development have been made increasingly on hydrogen absorbing alloys suitable for use as an active material in batteries, particularly sealed Ni-hydrogen rechargeable batteries and various hydrogen absorbing alloys have been proposed as described, for example, in Nos. JP-A-61-45563 and JP-A-60-241652.
However, none of the hydrogen absorbing alloys thus far proposed does always satisfy all the characteristics required for as the active material for the negative electrodes in sealed Ni-hydrogen rechargeable batteries, and therefore further development is now desired.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a hydrogen absorbing alloy satisfying the above-described characteristics or requirements.
Another object of the present invention is to provide a sealed Ni-hydrogen rechargeable battery containing such a hydrogen absorbing alloy as an active material.
As the result of intensive research, it has now been found that use of tungsten and rather high content of iron gives rise to hydrogen absorbing alloys which meet the above requirements, and the present invention is based on the discovery.
The present invention provides a hydrogen absorbing Ni,Zr-based alloy comprising 5 to 20% by weight of titanium (Ti), 10 to 37% by weight of zirconium (Zr), 5 to 30% by weight of manganese (Mn), 0.01 to 15% by weight of tungsten (W), 6 to 30% by weight of iron (Fe), and balance nickel (Ni) and unavoidable impurities.
Also, the present invention provides a sealed Ni-hydrogen rechargeable battery comprising a negative electrode provided with a hydrogen absorbing alloy as an active material, an Ni positive electrode, a separator and an alkaline electrolytic solution, wherein the hydrogen absorbing alloy is composed of a hydrogen absorbing Ni,Zr-based alloy comprising 5 to 20% by weight of titanium (Ti), 10 to 37% by weight of zirconium (Zr), 5 to 30% by weight of manganese (Mn), 0.01 to 15% by weight of tungsten (W), 6 to 30% by weight of iron (Fe), and balance unavoidable impurities.
The hydrogen absorbing Ni,Zr-based alloy of the present invention exhibits the above-described characteristics required for when used as an active material for the negative electrode of sealed Ni-hydrogen rechargeable batteries. The Ni-hydrogen rechargeable battery of the present invention has high energy density and high discharge capability as well as prolonged lifetime, shows low self discharge, realizes charge-discharge at high efficiency, causes no environmental pollution, and incurs low cost.